The invention has particular application in the machining of work surfaces by means of a gas plasma, in the protection of very clean surfaces and in applying a layer or coating to a surface. Such applications occur in many branches of the electrical, electronic and mechanical industries as well as in medical treatment, for example in a bacterial chamber.
Plasma machining methods and plants are known using controllable gaseous media in a special sealed chamber. The chamber is first pumped out completely and then filled with either a pure gas or a required mixture of gases. The plasma is activated by a high frequency, or ultra high frequency, glow or arc discharge which, depending on the composition of the gas mixture or the materials of the cathode or the anode, form the required composition of a plasma for cleaning, pickling or deposition of a film, (see for example V. I. Orlov and V. M. Garan, "Plasma Arc Spray Deposition of Coatings, Technology and Equipment", in the "Reviews of Electronic Equipment" series, "Technology, Production Management and Equipment", 1981 issue 18 (833), p. 46-47).
The chamber in such plants functions under low pressure. A higher vacuum tends towards a reduction of the unwanted impurities in the plasma. However, a higher vacuum also leads to a lower density of active particles and to a lower density of their flow towards the work surface which increases the duration of a treatment such as machining of the surface. This constitutes one of the reasons for a comparatively low output of such a plant. Furthermore, the low output is also a function of the need for a certain vacuum level to be continuously maintained, requiring the chamber to be pumped out. It is particularly important during the replacement of the plasma-generating gas that the chamber should be completely emptied of any particles in order to get rid of unwanted impurities that might cause problems during future machining of a work surface. This requirement entails prolonged preparation of the plant for operation. Furthermore, the use of gases containing aggressive active components, such as fluorine or chlorine, may induce reactions on the chamber walls, thus speeding up their wear. When such plants are used for a continuous machining processes, special sluice chambers have to be installed in order to enable there to be communication with the atmosphere. However, this requirement is liable to bring about contamination of a substrate which, for example in the field of electronics, is absolutely inadmissible.
A previously proposed method and apparatus for machining a solid body without a vacuum chamber, in which the work is introduced into a plasma flow and moved during machining in the plasma flow and then withdrawn over several cycles, has been described by B. S. Danilin, and V. Yu Kireyev in an article entitled "Employment of Low-Temperature Plasma for Pickling and Cleaning of Materials", 1987, Energatomizdat (Moscow) at pages 38-50. However, operating in this way renders a work surface liable to contamination and is quite unacceptable for use with ultra clean surfaces such as very large scale integrated circuits (VLSI circuits).
In yet another previously proposed arrangement mentioned in a publication entitled "Clean Atmosphere Premises" 1990 edited by I. Khayakava and published by MIR Publishers (Moscow) there is described at pages 58 and 59 a method of producing a laminar flow above a protected surface, the flow being produced from an ultra pure medium. The method is open, does not require a protective chamber, and can therefore be integrated comparatively easily into a continuous automated process. However, the method described is not sufficiently reliable for practical use, and is subject to the effects of turbulence.